1. Field of the Invention
The present invention relates to an air-fuel ratio computing apparatus for measuring and computing an air-fuel ratio of an internal combustion engine, and more particularly to an air-fuel ratio computing apparatus for suppressing deterioration of exhaust gas due to deviation of a calibration value for calibrating a theoretical air-fuel ratio voltage.
2. Description of the Related Art
In a correction device for a conventional air-fuel ratio detection apparatus, when a condition for stopping air-fuel ratio feedback control is determined based on various signals, a pump current flowing to a wide area type air-fuel ratio sensor is cut, output values of a circuit for transforming the pump current into voltage signals are measured more than once and averaged, a correction value is calculated by subtracting the average value of the output values from a standard voltage equivalent to the theoretical air-fuel ratio, and the newest computed value is stored (backed up) immediately before stopping engine operations. Then, when performing air-fuel ratio feedback control, at a time when the above-mentioned correction computation is not being performed, an air-fuel ratio detection value (digital value) is obtained from a conversion table, based on a corrected value produced by adding the above-mentioned correction value to the output value of the circuit for transforming the pump currents into the voltage signals (e.g., see JP 2001-221095 A (p. 1, FIG. 3)).
In the conventional apparatus as described above, when a voltage outputted from the air-fuel ratio detection apparatus passes through a 10-bit A/D converter, an A/D converted value always oscillates at an amplitude of several LSBs (meaning 2-3 bits at the smallest digit). Since the theoretical air-fuel ratio voltage is calibrated based on the A/D value, the calibrated theoretical air-fuel ratio voltage also has a width of several LSBs. This deviation of several LSBs in the theoretical air-fuel ratio voltage does not affect exhaust gas when running for a short time. However, when running for a long time, this small error accumulates, creating a problem of lowering a cleansing rate of a three-way catalyst.
Further, as a characteristic of a linear air-fuel ratio sensor, the amount of change in the air-fuel ratio per unit voltage is greater on a richer side of the theoretical air-fuel ratio than on a leaner side thereof. In other words, even if the voltage deviation from the theoretical air-fuel ratio is the same, the air-fuel ratio error will be greater when performing calibration toward the lean side, which causes a problem in that this has a great effect on the exhaust gas.